Magnesium alloys containing magnesium and various elements are lightweight and have a high strength-to-mass ratio and good shock absorbency. Therefore, magnesium alloys have been examined as constituent materials for housings of electric and electronic devices such as cellular phones and mobile computers and constituent materials for various members such as parts of automobiles. Since magnesium alloys have a hexagonal crystalline structure (hexagonal close-packed (hcp) structure), they have poor plastic formability at ordinary temperature. Therefore, magnesium alloy products used for the housings and the like are mainly formed of cast materials by a die casting process or a thixomolding process. However, when a thin sheet, in particular, the above-described member is mass-produced, it is difficult to produce a long sheet suitable for a raw material of such a thin sheet or member by the casting process above.
AZ31 alloy of the American Society for Testing and Materials (ASTM) standard is relatively easily subjected to plastic forming. Therefore, it has been examined that the thickness of a cast sheet composed of the AZ31 alloy is decreased by subjecting the cast sheet to plastic forming such as rolling or press forming. For example, Patent Literature 1 discloses that a thin magnesium alloy sheet is produced by subjecting a raw material composed of AZ31 alloy to warm rolling and then subjecting the raw material to shear deformation with a roller leveler and a recrystallization heat treatment in a combined manner.
Since AZ91 alloy of the ASTM standard has high corrosion resistance and strength, it is expected to be increasingly demanded as a wrought material. However, AZ91 alloy contains Al in a larger amount than AZ31 alloy and thus is inferior to AZ31 alloy in terms of plastic formability. Patent Literature 2 proposes that, when a magnesium alloy raw material sheet composed of AZ91 alloy and obtained by twin-roll casting or the like is subjected to rolling, the temperature of the raw material sheet and the temperature of a reduction roll be controlled in a certain range (relatively low temperature). As a result of the temperature control, an increase in the size of crystal grains is suppressed, cracks are not easily formed in the surface of the raw material, and rolling is properly performed.